The present invention relates to an improved microwave circuit and more particularly to an improved structure for providing tunable impedance matching for IMPATT diodes and similar devices.
IMPATT diodes are semiconductor devices which generate microwave power in GHz ranges. Their power output and efficiency are dependent upon the provision of a proper impedance match when operated as oscillators or amplifiers. Generally, since each diode will have characteristics different from any other diode, it is difficult to provide oscillators of reproducible characteristics or to maximize the operating characteristics of a circuit even when the diodes are produced by the same process. Because of the need for high power circuits in new technology systems, power coupling from diodes to transmission lines and other systems in oscillator and amplifier circuits has become more critical. As a result, various techniques have been proposed to couple IMPATT diodes and similar devices in circuits designed to improve their efficiency and power output.
In present systems, it is particularly desirable to combine a number of diodes to produce increased power output. Since no two IMPATT diodes are the same, the circuits necessary to combine the outputs are somewhat inefficient and complex. While there have been attempts to combine only diodes having similar characteristics, matching is difficult and tedious and is often abandoned at the expense of circuit performance. Other attempts have sacrificed the efficiency of diode operation by providing a greater number of diodes in a circuit so that a decrease in performance of a few diodes will not affect the overall operation of the circuit.
In other instances, impedance matching devices have been used to match the characteristics of a diode to a specific circuit. Microwave cavity and transmission line circuits have been used to provide a predetermined impedance match to an IMPATT diode. In such cases, however, the impedance cannot be changed and a separate impedance device must be used for each IMPATT diode. In other instance, a combining circuit is constructed with portions for matching impedance depending on the particular diode attached in the combining circuit. In these instances also, the combination can only receive diodes having identical characteristics in order for maximum power and efficiency to be achieved. Accordingly, if one of the diodes becomes inoperable in the combiner circuit, it must be replaced by an identical diode or the efficiency of the circuit will be decreased. The versatility of the coupler is therefore limited since it cannot accommodate diodes of different characteristics.
In addition to the above limitations, prior known devices are generally not acceptable for providing impedance matching in the miniaturized circuits of current technology. Transmission line and wave-guide matching techniques, for example, increase the size and weight requirements of a system where impedance matching is necessary. In many of the systems, the size and weight become critical and the transmission line and wave-guide coupling and impedance matching devices are unsuitable for use. Since present microwave systems employ a wide variety of power combining circuits coupled to utilize the power provided by IMPATT diodes and similar devices, there is a need for additional microwave impedance matching and tuning devices which interface with miniaturized circuits.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a simple and inexpensive microwave coupling circuit for matching and tuning impedance in IMPATT diode oscillator and amplifier circuits.